fix(backend): Fix top-up with zero transaction flow (#9886)

The transaction with zero payment amount will not generate a payment ID,
so the checkout failed for this scenario.

### Changes 🏗️

Don't use payment id as transaction key on top-up with zero payment
amount.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] Top-up with stripe coupon

autogpt_platform/backend/backend/data/credit.py

@@ -782,10 +782,15 @@ class UserCredit(UserCreditBase):
         # Check the Checkout Session's payment_status property
         # to determine if fulfillment should be performed
         if checkout_session.payment_status in ["paid", "no_payment_required"]:
-            assert isinstance(checkout_session.payment_intent, stripe.PaymentIntent)
+            if payment_intent := checkout_session.payment_intent:
+                assert isinstance(payment_intent, stripe.PaymentIntent)
+                new_transaction_key = payment_intent.id
+            else:
+                new_transaction_key = None
+
             await self._enable_transaction(
                 transaction_key=credit_transaction.transactionKey,
-                new_transaction_key=checkout_session.payment_intent.id,
+                new_transaction_key=new_transaction_key,
                 user_id=credit_transaction.userId,
                 metadata=Json(checkout_session),
             )